Crystalline boron nitride aerogels

ABSTRACT

This disclosure provides methods and materials related to boron nitride aerogels. For example, one aspect relates to a method for making an aerogel comprising boron nitride, comprising: (a) providing boron oxide and an aerogel comprising carbon; (b) heating the boron oxide to melt the boron oxide and heating the aerogel; (c) mixing a nitrogen-containing gas with boron oxide vapor from molten boron oxide; and (d) converting at least a portion of the carbon to boron nitride to obtain the aerogel comprising boron nitride. Another aspect relates to a method for making an aerogel comprising boron nitride, comprising heating boron oxide and an aerogel comprising carbon under flow of a nitrogen-containing gas, wherein boron oxide vapor and the nitrogen-containing gas convert at least a portion of the carbon to boron nitride to obtain the aerogel comprising boron nitride.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/320,300 filed Jun. 30, 2014, which claims priority to U.S. Provisional Application No. 61/841,716, filed Jul. 1, 2013, both of which are hereby incorporated by reference in their entireties.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy, under Contract No. DE-AC52-07NA27344 awarded by the U.S. Department of Energy, under Grant X10-8049-C awarded by the Air Force Office of Scientific Research. The government has certain rights in this invention.

BACKGROUND

Boron nitride (BN) forms bonding configurations similar to carbon. While sharing many of the same robust mechanical and thermal properties of graphite (Cohen et al., Phys. Today, 2010, 63:34-38; Dresselhaus et al., Philos. Trans. R. Soc., 2004, 362:2065-2098), the polar nature of the boron-nitrogen bond in planar (i.e., hexagonal) BN makes BN an optically transparent insulator (Robertson et al., Phys. Rev. B, 1984, 29:2131-2137), with different chemistry on the surface of the hexagonal lattice. As a consequence, BN-based materials are more resistant to oxidation than graphene-based materials (Chen et al., Appl. Phys. Lett., 2004, 84:2430-2432). BN also has enhanced physisorption properties due to the dipolar fields near its surface (Jhi et al., Phys. Rev. B, 2004, 69:245407). BN has been shown to surpass carbon in its ability to store gases such as hydrogen (Borek et al., Langmuir, 1991, 7:2844-2846; Li et al., Nanotechnology, 2013, 24:155603; M a et al., J. Am. Chem. Soc., 2002, 124:7672-7673; Kim et al., J. Mater. Chem. A, 2013, 1:1014-1017; Weng et al., ACS Nano, 2013, 7:1558-1565; Janik et al., Langmuir, 1994, 10:514-518), and is a very effective and reusable cleanup agent for hydrocarbons (Lei et al., Nat. Commun., 2013, 4:1777). Also, despite its relative chemical inertness, BN surfaces can be functionalized (Ikuno et al., Solid State Commun., 2007, 142:643-646; Lin et al., J. Phys. Chem. C, 2010, 114:17434-17439), allowing for their incorporation into composites (Zhi et al., Adv. Funct. Mater, 2009, 19:1857-1862), as well as their use as scaffolds for nanoparticles in catalysis and other applications (Sainsbury et al., J. Phys. Chem. C, 2007, 111:12992-12999).

Previous synthetic routes for BN-type aerogels and related meso-scale assemblies have included templated growth from existing porous structures such as zeolites (Schlienger et al., Chem. Mater., 2012, 24:88-96), gelation of colloidal BN suspensions (Jung et al., Sci. Rep., 2012, 2:849), and various high temperature reactions of boron and nitrogen containing compounds (Dibandjo et al., J. Eur. Ceram. Soc., 2007, 27:313-317; Paine et al., J. Inorg. Organomet. Polym., 1992, 2:183-195.). Despite a number of promising fields of application, research in this area has been slow due to the often toxic and volatile precursors involved and the limited synthetic routes to high quality BN. Furthermore, these synthesis methods have generally resulted in compounds of mixed BN phases and disordered or turbostratic stacking of planar sheets, corresponding to a crystal structure found in materials where atomic planes are inclined and rotated randomly with respect to one another.

Thus, a need exists for BN-based materials with improved structural quality.

SUMMARY

One aspect of the invention described herein relates to an aerogel material comprising boron nitride, the boron nitride having an ordered crystalline structure.

In some embodiments, the ordered crystalline structure includes atomic layers of hexagonal boron nitride laying on top of one another, and wherein atoms contained in a first layer are superimposed on atoms contained in a second layer.

In some embodiments, atomic planes of the boron nitride have an interplanar distance of about 3.3 Angstroms.

In some embodiments, the boron nitride described herein has a degree of crystallinity of at least 30%, or a degree of crystallinity of at least 40%, or a degree of crystallinity of at least 50%, or a degree of crystallinity of at least 70%, either by weight or by volume. In some embodiments, the boron nitride described herein does not have a substantially disordered crystalline structure.

In some embodiments, the aerogel described herein comprises sheets of boron nitride that are covalently bonded to one another. In some embodiments, at least 30% of the boron nitride sheets, or at least 40% of the boron nitride sheets, or at least 50% of the boron nitride sheets, or at least 70% of the boron nitride sheets, are covalently crosslinked with one another. In some embodiments, the aerogel consists essentially of sheets of boron nitride that are covalently bonded to one another.

In some embodiments, less than 50%, or less than 40%, or less than 30%, or less than 10% of the boron nitride sheets are associated with other boron nitride sheets by van der Waals force only. In some embodiments, the aerogel is substantially free of boron nitride sheets that are associated with other boron nitride sheets by van der Waals force only.

In some embodiments, the aerogel described herein comprises a three-dimensional network of sp²-bonded boron nitride. In some embodiments, at least 50% of the boron nitride, or at least 70% of the boron nitride, or at least 90% of the boron nitride, or at least 95% of the boron nitride, are sp²-bonded boron nitride.

In some embodiments, the aerogel described herein comprises at least about 70 atomic percent boron nitride, or at least about 80 atomic percent boron nitride, or at least about 90 atomic percent boron nitride, or at least about 95 atomic percent boron nitride. In some embodiments, the aerogel described herein consists essentially of boron nitride.

In some embodiments, the aerogel described herein comprises less than 10 atomic percent carbon, or less than 5 atomic percent carbon, or less than 2 atomic percent carbon, or less than 1 atomic percent carbon.

In some embodiments, the aerogel described herein has a surface area of about 350 meters squared per gram to 3000 meters squared per gram, or a surface area of about 350 meters squared per gram to 1050 meters squared per gram, or a surface area of about 1500 meters squared per gram to 3000 meters squared per gram.

In some embodiments, the boron nitride comprises hexagonal boron nitride.

In some embodiments, the aerogel comprises sheets of boron nitride, wherein the sheets of boron nitride include about 5 atomic layers of boron nitride or less. In some embodiments, the aerogel comprises sheets of boron nitride, wherein the sheets of boron nitride include an average of about 6-8 atomic layers of boron nitride.

In some embodiments, the aerogel comprises sheets of boron nitride, wherein at least a portion of the sheets of boron nitride form slit-shaped pore structures about 400 nanometers to 600 nanometers wide and about 2 microns to 4 microns long.

In some embodiments, the boron nitride has a white color.

In some embodiments, the aerogel has a mass density of about 50 milligrams per cubic centimeter to 150 milligrams per cubic centimeter.

In some embodiments, the aerogel comprises sheets of boron nitride, wherein the sheets of boron nitride have planar surfaces having dimensions of about 10 microns to 50 microns by about 10 microns to 50 microns.

In some embodiments, pores of the aerogel have diameters of about 2 nanometers to 50 nanometers.

Another aspect of the present invention described herein relates a method for making the aerogel material described herein, comprising: (a) providing boron oxide and an aerogel comprising carbon; (b) heating the boron oxide to melt the boron oxide and heating the aerogel; (c) mixing a nitrogen-containing gas with boron oxide vapor from molten boron oxide; and (d) converting at least a portion of the carbon of the aerogel to boron nitride.

In some embodiments, the aerogel comprising carbon in (a) includes graphitic carbon. In some embodiments, the aerogel comprising carbon in (a) is a graphene aerogel. In some embodiments, at least 50%, or at least 70%, or at least 90% of the graphene sheets are chemically crosslinked to other graphene sheets by covalent bonds. In some embodiments, less than 50%, or less than 30%, or less than 20%, or less than 10% of the graphene sheets are physically associated with other graphene sheets by van der Waals force only.

In some embodiments, at least about 70 atomic percent of the carbon, or at least about 80 atomic percent of the carbon, or at least about 90 atomic percent of the carbon, or at least about 95 atomic percent of the carbon is converted to boron nitride in (d).

In some embodiments, the method is substantially free of the use of additional promoters such as metal oxides for converting carbon to boron nitride. In some embodiments, the method does not comprise a step of burning off residual carbon.

The ordered crystalline structure described herein can comprise, for example, perfectly order crystalline structure as well as crystalline structure that differ slightly from perfectly order crystalline structure, such as where atoms in a first layer are displaced relative to atoms in a second layer by no more than 4 Angstroms, or no more than 3 Angstroms, or no more than 2.5 Angstroms, or no more than 2 Angstroms.

These and other features, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Note that the relative dimensions of the following figures may not be drawn to scale.

FIG. 1 shows an example of a method for synthesizing an aerogel comprising boron nitride.

FIG. 2 shows an example of a photograph of a graphene aerogel.

FIG. 3 shows an example of a photograph the graphene aerogel after conversion to a BN aerogel.

FIG. 4 shows an example SEM image of a BN aerogel. The structure is composed of many ultrathin, wrinkled sheets several micrometers long and folded into each other, forming slit-shaped macropores. Scale bar is 5 μm.

FIG. 5 shows example TEM images of original graphene aerogel (a, c) and converted BN aerogel (b, d). The lower-magnification TEM images (a, b) show that both aerogels are porous with feature sizes of about 30 nm. The higher-magnification TEM images (c, d) show that the aerogels comprise layered structures, indicated by the parallel, dark fringes. Upon conversion to BN, the layered structures become more crystalline with sharp transitions between the facets (d), compared to the more meandering layers observed in the graphene aerogels (c). In addition, the average number of wall for the constituent sheets increases from 2 to 3 in the case of the graphene gels to 6 to 8 for the BN gels. Scale bars are 200 nm (a, c) and 10 nm (c, d).

FIG. 6 shows an example high-resolution TEM image of a cross-link in a BN aerogel. The atomic layers of BN extend uninterrupted from all three aerogel sheets, showing that these extended structures are held together by covalent sp² bonds. Similar structures of varying geometries are found throughout the BN aerogel samples. Scale bar is 5 nm.

FIG. 7 shows (a) Example electron energy loss spectrum taken from a converted BN aerogel. The strong peak near 200 eV is attributed to boron, with edge features consistent with sp²-bonded BN; the peak near 400 eV is attributed to nitrogen. Noticeably absent is any distinguishable feature near 290 eV which would indicate the presence of carbon. The calculated ratio of boron to nitrogen is nearly 1:1, and the carbon concentration is below the resolution of the spectrometer (<5%). (b) Example Raman spectrum of the BN aerogel (solid line) and the graphene-based precursor (dotted line). The graphene aerogel spectrum shows broad peaks for the D and G bands in graphene, as observed in previous reports of graphene aerogel synthesis. The BN spectrum shows a single sharp peak at 1366 cm⁻¹, indicating that the BN aerogels have highly crystalline sp² bonding.

DETAILED DESCRIPTION

Reference will now be made in detail to some specific examples of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Particular example embodiments of the present invention may be implemented without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention will sometimes be described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless noted otherwise.

An aerogel is typically defined as a porous, monolithic, solid material comprised of cross-linked elements or constituents, whose pores are filled with air or some other gas. Aerogels typically have extremely low densities (e.g., about 0.0011 grams per cubic centimeter (g/cm³) to 0.5 g/cm³, or about 0.2 g/cm³) and high specific surface areas (e.g., about 100 meters squared per gram (m²/g) to 3000 m²/g). An aerogel is typically open-porous (i.e., the gas in the aerogel is not trapped inside solid pockets) and may have pores or slits about 1 nanometer (nm) wide to 1 micron wide.

Disclosed herein are synthesis methods and the characterization of high specific surface area (SSA), low density BN aerogels that in some embodiments are comprised locally of few atomic-layer sheets of hexagonal BN. In some embodiments, the BN aerogels show a high degree of crystalline order and chemical purity. In some embodiments, the synthesis methods use only common, non-hazardous reactants. In some embodiments, carbon nanostructures can be substantially or completely converted to BN while maintaining their macro and meso-scale morphologies.

In some embodiments, BN aerogel synthesis methods are based on the carbothermal reduction of graphene aerogels to BN aerogels having a similar structure as the graphene aerogels. The graphene aerogels, with a typical mass density of about 60 mg/cm³ to 150 mg/cm³, may be prepared using a number of different methods. In some embodiments, the graphene aerogels are placed together with boron oxide powder in a graphite crucible and heated in an induction furnace under nitrogen flow. At sufficiently high temperatures, the graphene sheets of the starting aerogel may be converted to hexagonal (hBN) nominally according to the following reaction:

B₂O₃+3C+N₂→2BN+3CO   (1)

The boron oxide is believed to react purely in the vapor phase, and BN is found to be the only solid product at temperatures above about 1500° C. in some embodiments. In practice, the operating temperature for this reaction is in the range of about 1600° C. to 1800° C. in order to provide sufficient vapor pressure of boron oxide. In some embodiments, contact between the aerogel and liquid boron oxide is avoided, so as to prevent any potential damage to the pore structure from capillary forces. The carbon and BN here are in their hexagonal phases; as such, reaction (1) represents an in-situ reforming of the sp² bonded layers of the starting material.

FIG. 1 shows an example of a method for synthesizing an aerogel comprising boron nitride. In operation 105 of a method 100, boron oxide and an aerogel comprising carbon are provided. In some embodiments, the boron oxide and the aerogel comprising carbon are placed in a graphite crucible (e.g., about 10 grams of boron oxide in an about 2″ diameter by about 4″ tall cylindrical graphite crucible). In some embodiments, the aerogel is supported in the middle of the crucible by a graphite cup with holes drilled in the bottom to allow for flow of reactant gases.

The aerogel comprising carbon may be fabricated using many different techniques. For example, in some embodiments, graphene oxide powder (e.g., about 0.2 grams) is mixed with deionized water (e.g., about 16.7 mL) in a glass vial and sonicated to reach a smooth liquid consistency. Ammonium hydroxide (e.g., about 3.3 milliliters, about 28 weight percent solution) is added and mixed well. The vial is sealed and heated to about 85° C. for about 40 hours, which produces a monolithic hydrogel. The gel is then soaked sequentially in deionized water and high purity isopropanol alcohol. Solvent is removed by supercritical CO₂ drying in a critical point dryer. The aerogels are then graphitized via firing for about 3 hours at about 1100° C. under an argon gas flow. Further details regarding this process for fabricating a carbon aerogel can be found in U.S. patent application Ser. No. 13/204,277 and in the publication Worsley, M.; Pauzauskie, P.; Olson, T.; Biener, J.; Satcher, J.; Baumann, T. Synthesis of graphene aerogel with high electrical conductivity. J. Am. Chem. Soc. 2010, 132, 14067-14069, both of which are herein incorporated by reference in their entireties.

In operation 110, the boron oxide is heated to melt the boron oxide and the aerogel is heated. For example, a crucible containing the boron oxide and the aerogel may be heated to melt and evaporate the boron oxide, and to heat the aerogel and any gases contained in the crucible. In some embodiments, the boron oxide and the aerogel are heated with a radio frequency induction furnace. In some embodiments, the boron oxide and the aerogel are heated to about 1600° C. to 1800° C.

In operation 115, a nitrogen-containing gas is mixed with boron oxide vapor from molten boron oxide. In some embodiments, the nitrogen-containing gas comprises nitrogen. In some embodiments, the flow rate of the nitrogen-containing gas is about 1000 standard cubic centimeters per minute (sccm) to 2000 sccm, or about 1500 sccm. In some embodiments, the nitrogen-containing gas is introduced through a central tube of a cylindrical graphite crucible, which may aid in transporting boron oxide vapor upward towards the aerogel.

In operation 120, at least a portion of the carbon of the aerogel is converted to boron nitride. In some embodiments, the conversion may be performed in about 30 minutes or less at a temperature greater than about 1600° C.; performing operation 120 in about 30 minutes or less may preserve the nanostructure of the aerogel during the conversion process. At 1600° C. to 1800° C. and using nitrogen as the nitrogen-containing gas at a flow rate of 1500 sccm, the evaporation rate of boron oxide is about 500 mg per minute. Further details regarding the method 100 may be found in U.S. Provisional Patent Application No. 61/751,641, which is herein incorporated by reference in its entirety.

After performing the method 100, the aerogel comprises boron nitride. In some embodiments, the boron nitride has a substantially ordered crystalline structure. In some embodiments, the aerogel comprises at least about 95 atomic percent boron nitride. In some embodiments, the ordered crystalline structure includes atomic layers of hexagonal boron nitride lying on top of one another, with atoms contained in a first layer being superimposed on atoms contained in a second layer. For example, the atomic layers of hexagonal boron nitride may be stacked such that boron atoms of a first layer overlay nitrogen atoms of a second layer, and the nitrogen atoms of the first layer overlay boron atoms of the second layer. In some embodiments, the boron nitride comprises hexagonal boron nitride. In some embodiments, the boron nitride has a white color. In some embodiments, atomic planes of the boron nitride have an interplanar distance of about 3.3 Angstroms.

In some embodiments, the boron nitride does not have a substantially turbostratic crystalline structure (i.e., a type of crystal structure found in materials where atomic planes, such as those in graphite or BN, are inclined and rotated randomly with respect to one another). In some embodiments, the boron nitride does not have a substantially disordered crystalline structure.

In some embodiments, the aerogel comprises sheets of boron nitride that are covalently bonded to one another. In some embodiments, there is overlap between the sheets of boron nitride of the aerogel, which may increase the mechanical stability of the aerogel while maintaining a high specific surface area. In some embodiments, atomic planes of BN may be shared between neighboring sheets of BN, which may further increase the mechanical stability of the aerogel. When forming an aerogel comprising boron nitride using other processing techniques, and not the carbothermal reduction technique described herein, the aerogel may include small particles of boron nitride (e.g., typically boron nitride crystallites tens of atomic layers thick) that are weakly bound together with little or no overlap (i.e., few of no particles sharing an atomic plane of boron nitride) or covalent bonding between the particles of the aerogel.

In some embodiments, the aerogel comprises sheets of boron nitride, with the sheets of boron nitride including about 5 atomic layers of boron nitride or less. In some embodiments, the aerogel comprises sheets of boron nitride, with the sheets of boron nitride including an average of about 6 atomic layers of boron nitride. In some embodiments, the aerogel comprises crumpled sheets of boron nitride, with the sheets of boron nitride having a thickness and dimensions of about 10 microns to 50 microns by about 10 microns to 50 microns.

In some embodiments, the aerogel comprises sheets of boron nitride, with at least a portion of the sheets of boron nitride forming slit-shaped pore structures about 400 nanometers to 600 nanometers wide and about 2 microns to 4 microns long. In some embodiments, the aerogel comprises sheets of boron nitride, with at least a portion of the sheets of boron nitride forming slit-shaped pore structures about 500 nanometers wide and about 3 microns long.

In some embodiments, the aerogel has a specific surface area of about 350 meters squared per gram (m²/g) to 3000 m²/g. In some embodiments, the aerogel has a specific surface area of about 350 m²/g to 1050 m²/g. In some embodiments, the aerogel has a specific surface area of about 700 m²/g. In some embodiments, the aerogel has a specific surface area of about 1500 m²/g to 3000 m²/g. In some embodiments, the aerogel has a mass density of about 50 milligrams per cubic centimeter (mg/cm³) to 150 mg/cm³. In some embodiments, the aerogel has a mass density of about 100 mg/cm³. In some embodiments, pores of the aerogel have a diameter of about 2 nanometers to 50 nanometers.

Potential applications of boron nitride aerogels include water purification, gas storage (including hydrogen and CO₂), oil recovery from oil spills, catalyst support, gas sensor, biosensor, fuel cell membranes, batteries, filters, poison control, and supercapacitors.

These and other features, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.

Working Examples

The following description of the characterization of a BN aerogel synthesized using methods disclosed herein and of the reactions associated with BN aerogel synthesis methods are intended to be examples of the embodiments disclosed herein, and are not intended to be limiting.

EXAMPLE 1 Methods

Synthesis of Graphene Aerogels. Graphene oxide powder (Cheap Tubes Inc., 0.2 g) was mixed with deionized water (16.7 mL) in a 20 mL glass vial and sonicated to reach a smooth liquid consistency. Ammonium hydroxide (3.3 mL, 28 wt % solution) was added and mixed well. The vial was sealed and heated at 85° C. for 40 h, resulting in a monolithic hydrogel. The gel was then soaked sequentially in deionized water and high-purity isopropyl alcohol. Solvent was removed by supercritical CO₂ drying. The aerogels were then graphitized via firing for 3 h at 1100° C. under argon flow.

Conversion of Graphene Aerogels. The conversion occurred in a 5 cm diameter by 10 cm tall cylindrical graphite crucible. The aerogels were supported in the middle of the crucible by a graphite cup with holes drilled in the bottom to allow for proper flow of the reactant gases. About 10 g of boron oxide powder (Alfa Aesar A11707) was placed at the bottom of the crucible, which was then heated under nitrogen flow (1500 sccm) in a radio frequency induction furnace to between 1600° C. and 1800° C. The powder was premelt to eliminate adsorbed water to prevent overbubbling and damaging the sample. Nitrogen was introduced through a central tube which in turn mixed well with the boron oxide vapor and helped to carry it upward toward the aerogels. The conversion was run long enough to allow for all of the boron oxide to evaporate; under these conditions, the evaporation rate of boron oxide was between 200 and 500 mg/min.

Determination of Interlayer Spacing. Starting from a high-resolution TEM image, the grayscale values of the pixels along a line transecting the given fringes were plotted. For two to three fringes, the resulting curve was fitted to a set of Gaussians using a Levenberg-Marquardt algorithm. For four or more fringes, a discrete Fourier transform was taken using a fast Fourier transform (FFT) algorithm. The resulting spectrum displayed a distinct Fourier peak superimposed on a 1/f background. The quoted values and precision for interplanar spacing corresponded to the center and width of the peak, respectively.

Characterization. TEM images were collected on a JEOL JEM2010 microscope operating at 80 kV. Samples were prepared by suspending the materials in isopropyl alcohol via ultra-sonication and then drop-casting onto holey carbon grids; alternately, the grids were simply rubbed gently against a cleaved surface of the gel. EELS was performed using a Phillips CM200 TEM operating at 200 kV and equipped with a Gatan imaging filter. Raman spectra were collected on a Renishaw inVia spectrometer using a 633 nm excitation laser. Nitrogen adsorption isotherms were measured using a Micromeritics ASAP 2010 porosimeter. XRD was performed using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation.

EXAMPLE 2 Results

FIG. 2 shows an example of a photograph of a graphene aerogel. FIG. 3 shows an example of a photograph the graphene aerogel after conversion to a BN aerogel. The BN aerogel substantially maintained its original size and macroscopic shape. There was, however, a dramatic change in color from black to white, which was uniform throughout the cross-section of the sample. There was a small degree of mass loss due to the boron nitride conversion process (i.e., about 10%), which is consistent with other studies of carbothermal reduction of boron nitride.

Scanning electron microscopy showed that the meso-scale BN aerogel had a foliated architecture very similar to that of the starting graphene aerogel, as shown in FIG. 4. Thin sheets several tens of microns on a side were found crumpled together, forming slit-shaped pore structures around 500 nanometer (nm) wide and several microns long. These structures were stable, despite being atomically thin, owing to the relatively high stiffness of few-layer BN sheets. Graphene and BN are unique in forming such structures; the mechanical properties of the sheets, coupled with the low atomic numbers of their constituent atoms, allows for the fabrication of crystalline, high SSA materials.

Transmission electron microscopy of graphene aerogels (FIG. 5a, 5c ) and converted boron nitride aerogels (FIG. 5b, 5d ) showed that the morphology on the 100 nm scale was not significantly altered upon conversion to BN. In both the graphene and the BN aerogels, the material was composed of wrinkled sheets with facets on the order of hundreds of nm² and several atomic layers thick. However, differences were apparent at higher magnifications (FIG. 5c, 5d ). The intrinsic “sheets” of converted BN, while retaining roughly the same size and shape, were approximately twice as thick as those of the original graphene aerogel, increasing from 3 to 6 atomic layers on average. These thicker BN sheets were straighter and more ordered than those of the graphene aerogel; e.g., in some cases, atomically straight edges 40 nm and longer were observed, whereas such features were at most 10 nm in the graphene aerogel precursor.

FIG. 6 shows a high-magnification TEM image of a cross-link in a converted BN aerogel. The sheets here formed a “Y” junction, wherein neighboring sheets shared one or more atomic layers. Some layers extended fully from one sheet to the next, while others overlapped by several nanometers. Such junctions were found in regions of the sample where two sheets laid atop one another. The aerogel thus appeared to be an interlaminated structure, where shared, sp²-bonded layers of BN form the cross-links between the sheets.

The chemical composition and bonding structure of BN aerogels was analyzed via electron energy loss spectrum (EELS) taken over a probe area of about 50 nm radius, as shown in FIG. 7a . A distinct nitrogen K edge at 401 eV, as well as the well-resolved boron peaks arising from π* and σ* states at around 188 eV, indicated a well-ordered, hexagonally bonded BN. The absence of any carbon K edge at 284 eV confirmed that the original graphene aerogel had been completely consumed in the conversion process. The boron to nitrogen atomic ratio for this sample was 0.97±0.14, while the carbon content was less than 5%.

The dramatic change in the chemical composition and crystalline order upon converting graphene aerogel to BN aerogel was also evident in the Raman spectra of the respective materials, as shown in FIG. 7b . For the graphene aerogel, a strong D peak at 1329 cm⁻¹ and the absence of a significant 2D signal (not shown) indicated a multilayer, disordered graphene structure, as observed previously for other graphene aerogels synthesized by a similar method. After subtraction of a background signal, the asymmetric G band was fit into two Lorentzians at 1573 cm⁻¹ and 1601 cm⁻¹ with full width at half maximum (FWHM) of 60.56 cm⁻¹ and 26.74 cm⁻¹, respectively. This peak fitting was qualitatively similar to double-peak features found in graphene nanoribbons. After conversion to BN, these peaks were entirely absent and replaced by a single peak at 1366 cm⁻¹, as also observed in highly crystalline samples of pyrolytic h-BN. This E_(2g) peak was due to the same in-plane phonons that give rise to the G peak in graphene. There was no measurable shift in the peak, as is expected for BN sheets more than 5 atomic layers thick. The FWHM of 14 cm⁻¹ indicated a highly crystalline BN and the small increase in peak width from 9.1 cm⁻¹ was likely related to finite phonon lifetimes due to the nanoscale crystal grains within the wrinkled sheets of the aerogel. A quantitatively similar broadening of the E_(2g) peak has been seen previously in BN crystallites similar in size and shape to the aerogel platelets seen in the TEM images.

The increase in crystalline order, as well as the increase in the wall thickness compared to the graphene precursor, have also been seen in the conversion of other nanostructured carbon materials, and appear to be a consequence of the specifics of the conversion reaction. The doubling of the number of layers making up the BN aerogel was corroborated by an approximate halving of the SSA; for one particular sample, the mesopore surface area (2 nm to 50 nm pore diameter) derived from nitrogen adsorption isotherms and calculated using the Brunauer-Emmett-Teller (BET) model was reduced from 1390 m²/g for the graphene precursor to 675 m²/g for the BN aerogel. At the same time, the mass and consequently the density tended to decrease by around 10%. While Eq. (1) would imply a mass increase, it is possible that additional mass losses are introduced through impurity oxidizers as well as the formation of cyanogens.

The mass loss and SSA reduction, coupled with the observation that the nanoscale folds in the sheets are preserved, suggests a particular mechanism for the carbothermal reduction process. Namely, it seems likely that the BN conversion process proceeds via a face-to-face reaction of two graphitic surfaces. A possible intermediate step in reaction (1) is the inclusion of boron radicals in each of two adjacent graphene lattices (C_(n)) via a partially reduced boron oxide molecule, followed by nitridation:

B₂O₂+2C_(n)→2B*C_(n−1)+2CO   (2)

2B*C_(n−1)+2N₂→2BN+2C_(n−2) +C ₂N₂   (3)

It was observed (in separate experiments) that single-walled carbon nanotubes do not tend to survive the conversion process, whereas double-walled carbon nanotube do tend to survive the conversion process, which further supports this view.

The role of water, which is always present in the conversion system that was used (i.e., a cylindrical graphite crucible) because of the hygroscropic nature of the insulation in the furnace, should also be considered. It is known, for example, that the presence of water dramatically increases the formation of cyangens in the direct nitridation of graphite.

Further studies, in particular analysis of flue gases and the role of water concentration in the reactants, will help to clarify the details of the reduction process.

Finally, it should be noted that formation of BN via carbothermal reduction is typically surface-passivated, and thus conventionally additional promoters such as various metal oxides are necessary to obtain appreciable conversion yield. This is not the case for the methods disclosed herein. The exceptionally high SSA of graphene aerogels lends itself particularly well to the conversion process, and as such, no additional promoters are necessary to take the reaction to completion.

EXAMPLE 3 Conclusion

Disclosed herein are embodiments of an inexpensive and non-toxic process for the conversion of a porous crystalline graphene material to a similarly structured BN material, opening possibilities in the research and development of new nanostructured BN materials. The growing number of applications for porous BN demands new methods for synthesis. The processes disclosed herein show that the meso-scale architecture, as well as the nanoscale morphology of the porous precursor materials, can be well maintained through conversion to BN.

Further details regarding some of the example embodiments disclosed herein can be found in Rousseas et al., Synthesis of Highly Crystalline sp²-Bonded Boron Nitride Aerogels, ACS NANO, 2013, 7(10):8540-8546, which is herein incorporated by reference in its entirety.

As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a marker can include multiple markers unless the context clearly dictates otherwise.

As used herein, the terms “substantially,” “substantial,” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, the terms can refer to less than or equal to ±10%, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

In the foregoing description, it will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations, which is not specifically disclosed herein. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention. Thus, it should be understood that although the present invention has been illustrated by specific embodiments and optional features, modification and/or variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scopes of this invention. 

What is claimed is:
 1. A method for making an aerogel comprising boron nitride, comprising: (a) providing boron oxide and an aerogel comprising carbon; (b) heating the boron oxide to melt the boron oxide and heating the aerogel; (c) mixing a nitrogen-containing gas with boron oxide vapor from molten boron oxide; and (d) converting at least a portion of the carbon to boron nitride to obtain the aerogel comprising boron nitride.
 2. The method of claim 1, wherein the aerogel comprising carbon comprises sp² carbon.
 3. The method of claim 1, wherein the aerogel comprising carbon is a graphene aerogel.
 4. The method of claim 1, wherein the boron oxide is heated to a temperature of at least about 1500° C.
 5. The method of claim 1, wherein the boron oxide is heated to a temperature of about 1600° C. to about 1800° C.
 6. The method of claim 1, wherein flow rate of the nitrogen-containing gas is about 1000 sccm to about 2000 sccm.
 7. The method of claim 1, wherein the nitrogen-containing gas is N₂.
 8. The method of claim 1, wherein the aerogel comprising carbon is substantially free of contact with liquid boron oxide.
 9. The method of claim 1, wherein at least about 95 atomic percent of the carbon is converted to boron nitride.
 10. The method of claim 1, wherein the aerogel comprising boron nitride obtained has an ordered crystalline structure comprising atomic layers of hexagonal boron nitride laying on top of one another, and wherein atoms contained in a first layer are superimposed on atoms contained in a second layer.
 11. A method for making an aerogel comprising boron nitride, comprising heating boron oxide and an aerogel comprising carbon under flow of a nitrogen-containing gas, wherein boron oxide vapor and the nitrogen-containing gas convert at least a portion of the carbon to boron nitride to obtain the aerogel comprising boron nitride.
 12. The method of claim 11, wherein the aerogel comprising carbon comprises sp² carbon.
 13. The method of claim 11, wherein the aerogel comprising carbon is a graphene aerogel.
 14. The method of claim 11, wherein the boron oxide is heated to a temperature of at least about 1500° C.
 15. The method of claim 11, wherein the boron oxide is heated to a temperature of about 1600° C. to about 1800° C.
 16. The method of claim 11, wherein flow rate of the nitrogen-containing gas is about 1000 sccm to about 2000 sccm.
 17. The method of claim 11, wherein the nitrogen-containing gas is N₂.
 18. The method of claim 11, wherein the aerogel comprising carbon is substantially free of contact with liquid boron oxide.
 19. The method of claim 11, wherein at least about 95 atomic percent of the carbon is converted to boron nitride.
 20. The method of claim 11, wherein the aerogel comprising boron nitride obtained has an ordered crystalline structure comprising atomic layers of hexagonal boron nitride laying on top of one another, and wherein atoms contained in a first layer are superimposed on atoms contained in a second layer. 